A load of an output transistor (main switching element) such as LDMOS (laterally diffused metal oxide semiconductor), for example, is sometimes short-circuited. When a driving signal for turning on the output transistor, which is short-circuited, an overcurrent exceeding an absolute maximum rated current flows in the output transistor. To counter this problem, an overcurrent protection circuit is provided as exemplarily disclosed in, for example, JP 2012-060437A. The overcurrent protection circuit includes a detection transistor (sensing switching element) and a driving limitation circuit. The detection transistor has a gate connected to a gate of the output transistor to supply a detection current Is, which corresponds to a current Im flowing in the output transistor. The driving limitation circuit draws a driving limitation current corresponding to the detection current Is from a current path, which extends from a gate driving circuit for driving the gate of the output transistor to the gate of the output transistor.
According to the conventional overcurrent protection circuit described above, the current flowing to the output transistor is limited in the following way when a load or the like is short-circuited. That is, when a driving signal for turning on is supplied to the output transistor with the load being short-circuited, the current Im rapidly increases. When the detection current Is increases to a value corresponding to the overcurrent detection threshold of the current Im, a gate capacitance is prevented from being charged because the driving limitation current is drawn from the current path extending to the gate of the output transistor. As a result, a gate-source voltage of the output transistor and hence the current Im are limited.
In the conventional overcurrent protection circuit described above, a current mirror circuit, which is formed of bipolar transistors for example, is used as the driving limitation circuit. The conventional overcurrent protection circuit thus has the following problems. That is, the overcurrent detection threshold varies with a forward voltage Vf of a transistor, which is connected in a current flow path of the detection current Is among transistors forming the current mirror circuit. For this reason, in the conventional overcurrent protection circuit, the overcurrent detection threshold varies due to a temperature characteristic of the forward voltage Vf. When the overcurrent detection threshold varies to delay a start of limiting the current Im, an overcurrent tends to flow to the output transistor and, in a worst case, causes thermal breakdown of the output transistor. When the overcurrent detection value varies to advance the start of limiting the current, the current Im flowing to the output transistor normally tends to be limited too early.
Further, in the conventional overcurrent protection circuit, the current mirror circuit does not operate until a voltage between main terminals of the output transistor reaches the forward voltage Vf. In this case, the driving limitation current is not drawn and hence the current Im is not limited. That is, in the conventional overcurrent protection circuit has a dead zone, in which the current Im cannot be limited. Because of the dead zone, the overcurrent detection threshold need be set high. With such a high threshold, the current Im cannot be limited timely and tends to break the output transistor down depending on a rate of change (slew rate) of the current Im at the time of an occurrence of a short-circuit failure. The dead zone tends to arise more as an on-resistance of the output transistor decreases, that is, as its size increases.
For the above-described reasons, when the conventional overcurrent protection circuit is used, the absolute maximum rated value of a drain current of the output transistor is designed to be far higher than a desired specification (designed to provide a large margin) thereby to avoid occurrence of the above-described problems. However, such a design adversely causes a problem that a chip area of the output transistor and hence an entire circuit area increases.